Grating guides for optical surface waves

ABSTRACT

Waveguides for use with guided elastic waves and guided optical waves wherein the lateral leakage rate of the waveguides is very small. The decreased leakage rate is effected by longitudinal strip regions or gratings periodically spaced on either side of the main waveguide region. These periodically spaced gratings rapidly decrease the rate of the evanescant portion of the wave according to a geometric law. Further, by controlling the amount of discontinuity presented by the periodic gratings the rate of evanescance may be controlled.

Ash

GRATING GUIDES FOR OPTICAL SURFACEWAVES [75] Inventor: Eric Albert Ash, London, England [73] Assignee: International Business Machines Corporation, Armonk, N.Y.

[22] Filed: July 20, 1972 [2]] Appl. No: 273,576

Related US. Application Data [62] Division of Ser. No. 203,185, Nov. 30, 1971,

abandoned.

[52] US. Cl 350/96 W6 [51] Int. Cl. G02b 5/14 [58] Field of Search 350/96 W0 [56] References Cited UNITED STATES PATENTS 3,381,246 4/1968 Perry 333/30 3,185,943 5/1965 Honda et a1. 333/72 3,444,482 5/1969 Becker 333/18 3,695,745 10/197 Furukawa 350/96 wo 3,465,159 9/1969 Stem 350/96 WG UX 3,563,630 2/1971 Anderson et al 350/96 WG 3,659,916 5/1972 Marcatili 350/96 WG 3,659,915 5/1972 Maurer et a1. 350/96 WG l g 7; J0

.Illl

405A 4058 403 H l I 1 Mar. 5, 1974 OTHER PUBLlCATlONS Heidrich et a1. Transducer Array for Generation of Multiple Surface-Wave Columns" lBM Technical Disclosure Bulletin Vol. 15, No. 1, June 1972, pp. 168,169.

Ash Tapered Grating Reflectors IBM Technical Disclosure Bulletin Vol. 13, No. 5, Oct. 1970, pp. 1204, 1205.

Primary Examiner-John K. Corbin Attorney, Agent, or FirmJohn D. Garvic 5 7] ABSTRACT Waveguides for use with guided elastic waves and guided optical waves wherein the lateral leakage rate of the waveguides is very small. The decreased leakage rate is effected by longitudinal strip regions or gratings periodically spaced on either side of the main waveguide region. These periodically spaced gratings rapidly decrease the rate of the evanescant portion of the wave according to a geometric law. Further, by controlling the amount of discontinuity presented by the periodic gratings the rate of evanescance may be controlled.

8 Claims, 6 Drawing Figures PATENTEDHAR M 3.795.434

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BY ATTORNEY PATENTED 51974 SHEET 2 0? 2 FIG. 4

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GRATING GUIDES FOR OPTICAL SURFACE WAVES This is a division, of application Ser. No. 203,185 filed Nov. 30, 1971, now abandoned.

FIELD OF THE INVENTION This invention relates to optical waveguides.

BACKGROUND OF THE INVENTION Elastic waves are used in a variety of signal processing devices. While the existence of elastic eaves waves been known for many years, only recently have advances in such technology as materials and fabrication permitted the practical implementation of elastic wave devices. One reason for the high interest in elastic waves resides in the fact that they are typically five orders of magnitude slower than electromagnetic waves. This much slower velocity of elastic waves enables transmission components such as filters, resonators, and delay lines to be constructed on a subminiature scale. The further development of devices such as amplifiers, detectors and modulators make elastic wave technology an extremely valuable tool in electronic signal processing.

Similarly, recent developments in materials and fabrication have also resulted in advances in optical guided wave technology. By using transparent dielectric film, optical waveguides are constructed upon the surface of a material. The use of optical guided waves has overcome many of the difficulties involved in regular optical communications such as the critical placement of prisms, mirrors, and bulky lenses.

Waveguide techniques used to guide elastic waves and optical waves are similar in many respects, and many techniques employed in one technology are applicable in the other. For example, both elastic waves and optical waves can be guided on the surface ofa material by either making th phase velocity characteristics slower or less than the phase velocity characteristics of the wave in the surrounding medium or by providing discontinuous boundaries on either side of the waveguide which will confine the wave within the waveguide. If, in the optical case, a metal boundary on either side of the waveguide is provided or, in the acoustic case, an infinitely rigid boundary on either side of the waveguide exists, confinement is complete. Alternately, if the wave velocity within the waveguide is markedly lower than the wave velocity of the surround ing region, the confinement is also complete in the sense that no energy is leaked" from the desired mode, even though much of the energy may be propagating outside the waveguide. In practice, however, there are several technological difficulties associated with either approach. It is difficult from a fabrication standpoint to provide a sufficiently marked discontinuity at the waveguide boundaries to confine the wave without resorting a velocity difierence inside and outside the regions. The technique used for lowering the t velocity within the waveguide has thus far resulted in lossy waveguides. They are prohibitively so at microwave frequencies. Further, since fabrication techniques have limited leakage rate reduction capabilities, the ability to vary the leakage rates have also been limited.

Accordingly, a principal object of this invention is to reduce the lateral leakage rate of optical waves traveling in optical waveguides and surface elastic waves traveling in surface waveguides in an improved manner.

It is another object of this invention to improve the control of the lateral leakage rate of waves in optical and surface waveguides.

SUMMARY OF THE INVENTION It has been discovered that the lateral leakage rate of surface elastic waves and optical waves propagating in waveguides may be significantly reduced by periodically spacing additional longitudinal discontinuities on either side of the waveguide approximately parallel thereto. These parallel longitudinal discontinuities are constructed in the same manner as the main waveguide and are spaced at distances approximately equal to integrals of the lateral width of the waveguide. The leak age rate becomes rapidly smaller as the number of discontinuities is increased, permitting the leakage rate to be controlled by selection of the number of discontinuities.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION FIG. 1 shows a topographic grating guide for acoustic surface waves. Theoretical considerations indicate that when the width W of guide 101 is wide relative to the Rayleigh wavelength, guide 101 can be operated under conditions when only surface wave leakage can occur. The surface wave leakage may be minimized by making the discontinuity at edges 102 and 103 of of guide 101 as extreme as possible. The two possible techniques of minimizing are to make guide 101 very high or to undercut guide 101. A third technique for minimization which is not technologically difficult has been discovered. This third minimization technique is to space periodic arrays of discontinuities or ridges 105 A-F on either side of guide 101. While presently the discontinuity offered by each step of the periodic array has not beem mathematically determined, experimental results indicate that the effect 'of the various periods of the gratings will be multiplicative. Thus, if the power leakage arising from the use of one slot is n, the power leakage from the use of m slots will reduce this leakage to n"'. With each of the ridges 105 A-F about one wavelength in height Q s of the order of several hundreds can be obtained, suggesting that the use of three ridges should make the leakage completely negligible. The exact shape of the slots should not matter greatly.

The presence of these ridges 105 A-F reduces the bandwidth of guide 101. However, it is important to note that acoustic surface waves are not very dispersive over the operating range of guide 101.

Theoretical indications further indicate that the periodicy P of ridges 105 A-F is approximately equal to the width W of ridge 101, i.e., P-W. This has the important consequence that none of the grating ridges 105 A-F on either side of the main guiding ridge 101 can support a low leakage wave. Thus, there are no problems associated with mode conversion into the neighboring channels. A second important point to note is that the smallest critical dimension is well within the current fabrication capabilities of the art and, thus, the resolution required is substantially easier to achieve than an interdigital transducer designed for the same frequency range. Moreover, there will be no critical tolerances on the ridge-to-valley width.

The topographic guide 101 of FIG. 1 offers the advantage of being made of a single material 109. This is particularly important at the higher operating frequencies because of the difficulty of finding two compatible, low-loss materials.

FIG. 2 shows a thin film grating guide for use with acoustic surface waves, where the gratings are provided by a deposited film which mass-loads the surface. Longitudinal regions or deposited layers 201 and 202 define the waveguide region 203 wherein acoustic surface waves propagate. Deposited layers 204 A-D function as periodic discontinuities in a similar manner as described for the topographic grating guide of FIG. 1. In principal. deposited layers 201, 202 and 204 A-D could have faster or slower shear velocity characteristics than substrate 205. The slower shear velocity characteristic is preferred, since deposited layers 201, 202 and 204 A-D are bound to perturb the velocity of the waves somewhat, in the direction of the velocity of deposited layers 201, 202 and 204 A-D. Thus, a fast film would bring the guide velocity closer to the shear velocity of substrate 205, making bulk reactions at bends and discontinuities hard to avoid. While deposited layers 201, 202 and 204 A-D with slow velocity characteristics act as guides, the guide velocity will be far removed from that of the main guide 203, which is very close to the Rayleigh velocity of substrate 205. Thus, cross-coupling problems are unlikely to be serious.

A slow-on-fast guiding structure in which deposited layers 201,202 and 204 A-D have a slower velocity characteristic than substrate 205, may be achieved with such materials as gold stripe for longitudinal regions 201, 202 and 204 A-D and fused silica for substrate 205. To achieve the opposite situation of a faston slow" guiding structure, an aluminum film for longitudinal regions 201, 202 and 204 A-D and T-40 delayline glass for substrate 205 may be used. If a strongly piezoelectric substrate is used, it is possible to realize a grating which relies soley on the reflections occasioned by shorting out the piezo field. Guides are then possible using very thin metallic films, so thin that the acoustirloss is quite negligible, even at microwave frequencies. A large number of strips would be required, perhaps of the order of 50 on either side of the guiding region 203. As a result, the guidance would be effective over a narrow frequency range only; however, as noted above, this narrow band of operation is not associated with high dispersion in the operating band. Further, resorting to more complicated gratings, it is possible to broad-band the device by tapering the periodicy.

FIG. 3 shows a doped grating for guiding acoustic surface waves. It is well known that acoustic velocity in a semiconductor is dependent upon the doping density.

For example. in germanium it is possible to obtain a change reduction of the shear velocity of 3 percent by doping. This velocity reduction could serve as a waveguide channel. These velocity reductions which can be obtained, through sizable, are less than preferred for a robust guiding system. Moreover, although it is known that some semiconductor materials, notably silica, have a relatively low acoustic loss, this applies only to the pure material. The losses in the doped material appear to be substantially higher.

For these reasons a grating guide as shown in FIG. 3 offers a number of significant advantages. First, smaller doping densities are sufficient. and secondly, most of the energy is propagating in the undoped region. The acoustic losses can, therefore, be expected to be substantially smaller. Finally, the width of the channel is substantially larger than would be required in a single channel guide.

Thus, in FIG. 3 longitudinal regions 301 and 302 define the guiding surface 303. Longitudinal regions 304 A-D are periodically spaced from longitudinal regions 301 and 302 in a similar manner as described for the topographical grating guide of FIG. 1. That is, the periodicy P is approximately equal to the width W of guiding surface 303. Longitudinal regions 301, 302 and 304 A-D are formed by appropriate doping of semiconductor substrate 305.

Because of the great number of similarities between techniques for handling acoustic surface waves and optical guided waves, the same principles which are used to explain the topographical grating guide of FIG. 1 are also applicable to various types of optical waveguides. FIG. 4 shows a deposited grating guide for optical guided waves. The thin layer 401, deposited upon substrate 402 has a refractive index which exceeds that of substrate 402 and the medium on the other side of thin layer 401, in this case air. Metallic gratings 403, 404 and 405 (A-D are evaporated onto layer 401, thus laterally confining optical waves which propagate in layer 401. Metallic gratings 403, 404 and 405 A-F function so as to change the effective impedance in the regions which are covered by the deposited metal. This impedance change will be significant only for the TM waves, so that the metal structure can only be used to guide this type of wave. However, if a dielectric film is deposited upon layer 401 rather than the metallic material, both TE and TM waves can be guided.

Longitudinal regions or grating structures 403 and 404 define the lateral boundaries for optical waves propagating in layer 401. Such a waveguide may be constructed using fused quartz for substrate 402 and alumina-doped silicon dioxide for thin layer 401. Gold may be used to construct longitudinal regions 403, 404 and 405 A-D.

FIG. 5 shows a deposited grating guide for optical guided waves in which the periodic variation in refractive index is electronically controllable. As in FIG. 4, a thin layer 501 is deposited upon substrate 502, thin layer 501 having a refractive index which exceeds that of substrate 502 and the medium on the other side of thin layer 501. Metallic gratings 503, 504 and 505 A-D are evaporated onto thin layer 501 since metallic gratings 503, 504 and 505 A-D functions to change the effective impedance of the regions over which they are deposited. The addition of controllable bias sources 506 and 507 connected to metallic gratings 503, 504 and 505 A-D. permit variable control of this effective impedance. Of course, thin layer 501 must be an electro-optical material for such control to be effected. This freedom to vary the strength of the grating effect enhances the tightness of the coupling and may also be used to construct a switchable directional coupler. The waveguide of FIG. 5 may be constructed of the same materials as suggested for the waveguide of FIG. 4. That is, fused quartz for substrate 502. alumina-doped silicon dioxide for thin layer 501, and gold for longitudinal region 503. 504 and 505 A-D. Variable voltage controls 506 and 507 can simply be direct current sources controlled by variable resistances in the circuitry.

FIG. 6 shows a doped grating guide for optical guided waves, the counterpart of the structure shown in FIG. 3 for acoustic surface waves. As in the acoustic case, doping changes the velocity of optical frequency electromagnetic waves in a semiconductor. An important point of difference is that in the optical guided wave, doping raises the velocity. It is. in fact, this phenomena which leads to the existence of guided modes in injection lasers. This phenomena also enables optical waves to be guided in high resistivity epitaxial layers on low resistivity substrates.

Thus, longitudinal regions or grating guides 601, 602 and 603 A-D are diffused or ion implanted onto thin layer 604. Thin layer 604, deposited on low resistivity substrate 605. confines the optical guided waves to region 606. Longitudinal regions 601, 602 and 603 A-D effect the impedance of thin layer 604, guiding the optical waves with greatly reduced lateral leakage.

OPERATION OF THE INVENTION The operation of the invention for acoustic surface waves can generally be described with reference to FIG. I. Transducer I06 couples acoustic surface waves onto surface 101. As these acoustic surface waves travel along surface 101 in the direciton indicated by arrow 107, their lateral leakage rate is reduced to negli gible proportions as a result of the periodic discontinuities presented by the periodically spaced grating I05 A-F. Finally. the acoustic surface wave reaches a second transducer 108, which changes the acoustic surface wave in electrical signal output.

Similarly. the operation for optical guided waves may generally be described with reference to FIG. 4. Optical guided waves are coupled into the waveguide by means of transducer 406. Thin layer 401 confines the optical guided waves to the surface of substrate 402 and longitudinal regions 403, 404 and 405 A-D reduce the lateral leakage rate of the optical waves. confining them to region 407 of thin layer 401. These optical waves are guided in a direction of arrow 408 and when they reach transducer 409. are coupled out of the optical waveguide.

Of course. once the optical guided wave or the acoustic surface wave has been coupled onto the waveguide, they may perform various functions before they are coupled out. However, this invention is primarily concerned with the configuration of the waveguides themselves.

ADVANTAGES From the above detailed description, the advantages of this invention are now readily apparent. The lateral leakage rate of both optical waves traveling in optical waveguides and acoustic surface waves traveling in acoustical waveguides is significantly reduced by spacing periodic gratings on either side of the waveguide region. These periodic gratings which represent lateral discontinuities to the waves are spaced at a distance approximately equal to the width of the guiding region. The amount of leakage reduction is controllable by the specific number ofgratings selected. Further in the optical guided wave case, control of the leakage rate may also be effected electronically.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those of skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. For example, other functions may be accomplished using the grating guide technique, such as directional couplers.

I claim:

1. A structure for guiding waves comprising:

a bulk material with a first refractive index having at least one surface;

a first layer upon the surface of said bulk material, with a second refractive index greater than said first refractive index. wherein optical waves are maintained, said layer having an exterior surface;

an input means for coupling said optical waves into said layer;

an output means coupling said optical waves out of said layer;

a first longitudinal region in said layer being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical waves; and

a plurality of second longitudinal regions approximately parallel to said first longitudinal region and periodically spaced from said first longitudinal region, each second longitudinal region being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical wave;

said longitudinal edges of said first and said plurality of second longitudinal regions being delineated by metal strips deposited on said exterior surface of said first layer;

wherein said periodicy of said plurality of second longitudinal regions is approximately equal to said lateral extent of said first longitudinal region.

2. The structure of claim 1 wherein an even number of said plurality of second longitudinal regions are equally divided on either side of said first longitudinal region.

3. The structure of claim 1 wherein said first layer is an electro-optic material: and

said structure further comprises a plurality of controllable voltage sources connected to each of said metallic strips, for varying the effective impedance presented by said metallic strips.

4. The structure of claim 3 wherein an even number of said plurality of second longitudinal regions are equally divided on either side of said first longitudinal region.

5. A structure for guiding waves comprising:

a bulk material with a first refractive index having at least one surface; t

a first layer upon the surface of said bulk material,

with a second refractive index greater than said first refractive index. wherein optical waves are maintained, said layer having an exterior surface;

an input means for coupling said optical waves into said layer;

an output means coupling said optical waves out of said layer:

a first longitudinal region in said layer being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical waves; and i a plurality of second longitudinal regions approximately parallel to said first longitudinal region and periodically spaced from said first longitudinal region. each second longitudinal region being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical waves;

said longitudinal edges of said first and said plurality of second longitudinal regions being delineated by dielectric strips deposited on said exterior surface of said first layer;

wherein said periodicy of said plurality of second longitudinal regions is approximately equal to said lateral extent of said first longitudinal region.

6. The structure of claim wherein an even number of said plurality of second longitudinal regions are equally divided on either side of-said first longitudinal region.

7. A structure for guiding waves comprising:

a bulk material with a first refractive index having at least one surface;

a first layer upon the surface of said bulk material,

with a second refractive index greater than said first refractive index. wherein optical waves are maintained. said layer having an exterior surface;

an input means for coupling said optical waves into said layer;

an output means coupling said optical waves out of said layer;

a first longitudinal region in said layer being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical waves; and

a plurality of second longitudinal regions approximately parallel to said first longitudinal regions and periodically spaced from said first longitudinal region, each second longitudinal region being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical waves;

said first layer being composed of a semiconductor material wherein said first longitudinal region and said plurality of second longitudinal regions are separated by said regions in said first layer doped with a semiconductor impurity;

wherein said periodicy of said plurality of second longitudinal regions is approximately equal to said lateral extent of said first longitudinal region.

8. The structure of claim 7 wherein an even number of said plurality of second longitudinal regions are equally divided on either side of said first longitudinal region. 

1. A structure for guiding waves comprising: a bulk material with a first refractive index having at least one surface; a first layer upon the surface of said bulk material, with a second refractive index greater than said first refractive index, wherein optical waves are maintained, said layer having an exterior surface; an input means for coupling said optical waves into said layer; an output means coupling said optical waves out of said layer; a first longitudinal region in said layer being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical waves; and a plurality of second longitudinal regions approximately parallel to said first longitudinal region and perIodically spaced from said first longitudinal region, each second longitudinal region being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical wave; said longitudinal edges of said first and said plurality of second longitudinal regions being delineated by metal strips deposited on said exterior surface of said first layer; wherein said periodicy of said plurality of second longitudinal regions is approximately equal to said lateral extent of said first longitudinal region.
 2. The structure of claim 1 wherein an even number of said plurality of second longitudinal regions are equally divided on either side of said first longitudinal region.
 3. The structure of claim 1 wherein said first layer is an electro-optic material; and said structure further comprises a plurality of controllable voltage sources connected to each of said metallic strips, for varying the effective impedance presented by said metallic strips.
 4. The structure of claim 3 wherein an even number of said plurality of second longitudinal regions are equally divided on either side of said first longitudinal region.
 5. A structure for guiding waves comprising: a bulk material with a first refractive index having at least one surface; a first layer upon the surface of said bulk material, with a second refractive index greater than said first refractive index, wherein optical waves are maintained, said layer having an exterior surface; an input means for coupling said optical waves into said layer; an output means coupling said optical waves out of said layer; a first longitudinal region in said layer being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical waves; and a plurality of second longitudinal regions approximately parallel to said first longitudinal region and periodically spaced from said first longitudinal region, each second longitudinal region being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical waves; said longitudinal edges of said first and said plurality of second longitudinal regions being delineated by dielectric strips deposited on said exterior surface of said first layer; wherein said periodicy of said plurality of second longitudinal regions is approximately equal to said lateral extent of said first longitudinal region.
 6. The structure of claim 5 wherein an even number of said plurality of second longitudinal regions are equally divided on either side of said first longitudinal region.
 7. A structure for guiding waves comprising: a bulk material with a first refractive index having at least one surface; a first layer upon the surface of said bulk material, with a second refractive index greater than said first refractive index, wherein optical waves are maintained, said layer having an exterior surface; an input means for coupling said optical waves into said layer; an output means coupling said optical waves out of said layer; a first longitudinal region in said layer being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical waves; and a plurality of second longitudinal regions approximately parallel to said first longitudinal regions and periodically spaced from said first longitudinal region, each second longitudinal region being bounded by longitudinal edges which provide sharp discontinuity in the effective impedance for said optical waves; said first layer being composed of a semiconductor material wherein said first longitudinal region and said plurality of second longitudinal regions are separated by said regions in said first layer doped with a semiconductor impurity; wherein said periodicy of said plurality of second longitudinal regions is approximately equal to said lateral extent of said first longitudinal region.
 8. The structure of Claim 7 wherein an even number of said plurality of second longitudinal regions are equally divided on either side of said first longitudinal region. 